Having a zombie process listed in your ps output is not a bad thing, necessarily. It's usually being kept around in the process table because the parent process may still need to read the exit status of the child process (zombied / defunct process). If it really bothers you, you can try to kill -9 the process. If that doesn make it go away, you can try to nudge the parent proicess by sending it a kill -HUP. To find the parent process:

ps axo ppid,stat | grep Z | awk '{print $1}' | xargs kill -HUP

or something similar if you like.

If you're a little bit familiar with C and UNIX programming environment, the following example might help you to understand what is a zombie process.

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

int main() {
    int pid;

     /* let's create a child process */
    pid = fork();
    if (!pid) {
         /* this is a child: dies immediately and becomes zombie */
        exit(0);
    }

     /* parent process: just asks for the user input */
     printf("Please, press enter after looking at the zombie process...");
     (void)getchar();
}